NASA said in a bulletin on its website that while the rate of ammonia leaking from the station's truss structure had increased, the "station continues to operate normally otherwise and the crew is in no danger."

"Ammonia is used to cool the station's power channels that provide electricity to station systems," NASA said, revealing that the leak was coming from the same general area it did in a previous episode last year.

"This ammonia loop is the same one that spacewalkers attempted to troubleshoot a leak on during a spacewalk on Nov. 1, 2012," the statement said.

"It is not yet known whether this increased ammonia flow is from the same leak, which at the time, was not visible."

The ISS is currently crewed by six staff, under the command of Canadian Chris Hadfield.